In recent years, the use of various pieces of content has been being increased over a network. In other words, various pieces of content, such as still images, moving images, and audio data (music tracks), are stored and managed in a content distribution server in the Internet or a server in an intranet or a home network, a user searches those pieces of content through a client device connected to the network, the client device receives desired content from the server, and the received content is output and is played back on the client side.
For example, when content, such as audio data or image data, stored in the server is played back on the client-side device, a typical processing sequence is as follows. First, an information acquisition request is transmitted from the client side to the server in order to acquire content information, such as the titles of music tracks or movies and artist names, and various pieces of content attribute information, such as data compression format information (e.g., ATRAC: adaptive transform acoustic coding or MPEG: moving picture experts group) and, as necessary, copyright information, the content information and the content attribute information being stored in the server. The attribute information is called as metadata or meta-information.
In response to the request from the client, the server transmits metadata (attribute information) relating to content stored in the server to the client. The client displays content information in a display of the client device according to a predetermined display program on the basis of the metadata acquired from the server. For example, a music title list including artist names and music track titles is displayed in the display. A user checks pieces of content to be played back or selects any one piece of content on the basis of the displayed information and then transmits a content transmission request to the server. The server receives a content request from the client and transmits content to the client in response to the received request. The received content is played back on the client side.
As data communication networks are becoming widespread, a home network is becoming popular. Home electric appliances, a computer, and other peripheral devices are connected through the home network in the home to realize communications between the devices. In the home network, the devices connected to the network communicate with each other such that data processing functions of the respective device are shared and content is transferred between the devices, thus providing convenience and amenity to the user. The home network is sure to be used even more widely in the future.
As a protocol suitable for the configuration of the home network, Universal Plug and Play (UPnP) is known. According to Universal Plug and Play (UPnP), a network can be easily established without complicated operations, and services provided by devices connected to the network are receivable in the connected devices without difficult operations and settings. UPnP does not depend on the OS (operating system) of each device. Advantageously, a new device can be easily connected to the network.
According to UPnP, the connected devices exchange definition files based on XML (extensible Markup Language) therebetween to recognize each other. General processes according to UPnP are as follows:
(1) An addressing process of obtaining its own device ID such as an IP address;
(2) A discovery process of retrieving the other devices connected to the network and receiving a response from each of the devices to obtain information, such as a device type and functions, included in the response; and
(3) A service request process of requesting a service to each device on the basis of the information obtained by the discovery process.
The above-mentioned processes are performed, so that services using the devices connected to the network can be provided and obtained. A device, which is newly connected to the network, obtains a device ID by the above-mentioned addressing process and further acquires information regarding the other devices connected to the network by the discovery process, so that the device can request a service to each of the other devices on the basis of the acquired information.
In some cases, simultaneously with a content distribution process of distributing content from a content providing server to a client, event notification is performed such that the server notifies the client of information related to content, e.g., various pieces of information related to content to be distributed, such as information regarding the start or end of content distribution and information regarding a change of a content distribution channel.
For example, in a system using the above-mentioned UPnP, an architecture called GENA (Generic Event Notification Architecture) is used as an event notification architecture. According to GENA, an event notification process of sending event information in HTTP (Hyper Text Transfer Protocol) from a content providing server to a client is executed. When there is any change in state variable of a service, such as a content distribution service, provided to a client by the server, a GENA-notify message is transmitted to the client to notify the client of the change of state. For example, Japanese Unexamined Patent Application Publication No. 2002-328851 (Patent Document 1) discloses a system for controlling audio visual devices utilizing GENA.
According to the event notification process, including the above-mentioned GENA, executed simultaneously with the common content distribution, fundamentally, the server individually notifies each client of an event. If many clients access one server to receive content, unfortunately, the load of the event notification process on the server becomes excessive. As events sent from the server increase, disadvantageously, network traffic also increases.